Coronary artery revascularization may be the most direct means of preserving ischemic myocardium after coronary occlusion, but the value of this therapy in the emergency management of preinfarction angina and acute myocardial infarction is limited by the necessity that revascularization be completed early in the time course of an ischemic episode, and by the relative intolerance of patients to major cardiac surgery during acute ischemia. Hypothermia has been proposed as a means for preoperative treatment to increase the time between acute infarction and effective surgical intervention. However some clinical findings suggest that whole body hypothermia might increase the incidence of intraoperative myocardial infarction during aorto-coronary bypass surgery. The goal of these studies is to determine what effect moderate whole body hypothermia has on the myocardial blood flow in and around an area of ischemic tissue produced by ligation of a coronary artery in the anesthetized open chest dog. The data should provide new insight into the evaluation of hypothermia as a method of protecting the heart from ischemic injury during coronary artery bypass surgery and acute myocardial infarction.